High-output fluorescent lamp having means for maintaining a predetermined mercury vapor pressure during operation



May 17, 1966 D. A. LARsoN ETAL 3,252,028

HIGH-OUTPUT FLUORESCENT LAMP HAVING MEANS FOR MINTAINING A PREDETERMINED MERCURY VAPOR PRESSURE DURING OPERATION Filed June 2s, 1961 FIG.

United States Patent O 3,252,028 HIGH-OUTPUT FLUORESCENT LAMP HAVING MEANS FOR MAINTAINING A PREDETER- MINED MERCURY VAPOR PRESSURE DUR- ING OPERATION Daniel A. Larson, Cedar Grove, and Grant W. Manning, Jr., Denville, NJ., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed .lune 23, 1961, Ser. No. 119,230 11 Claims. (Cl. 313-34) This invention relates to electric discharge devices and, more particularly, to fluorescent lamps of the high output or so-called highly-loaded type.

In order to provide more light at lower cost and maintenance there have recently been developed so-called high output uorescent lamps which have a much higher loading than lamps of conventional design. The term loading refers to the number of Watts per unit of lamp length. Thus, -a standard 40 =watt -uorescent lamp 4 foot in length is said to have a loading of watts per foot. Fluorescent lamps of conventional design having a loading as high as 16 watts per foot are known but are relatively ineicient as compared to lamps having a normal loading of 10 watts per foot and thus have not been widely adopted, at least not in this country. The high output lamps now -being marketed have loadings as high as 25 watts per foot.

As is well known, a conventional fluorescent lamp cannot be converted into a high output lamp simply by increasing the power input and thus increasing its loading. The reason for this is because the lamp elliciency decreases as the current density increases. Various means have been employed to compensate for the drop in eliiciency at high loadings, such as the use of indentations or grooves in the lamp envelope that increase the ion losses to the wall-s, and the use of lighter fill gases such as neon that increases the mobility of the mercury ions-both of which increase the diffusion losses and the electron temperature. This increase is electron temperature, in turn, increases the efficiency with which ultraviolet (UV) radiations'are produced within the discharge and, thus, the lamp eiciency.

ln addition to having a commercially acceptable eiliciency, a high output fluorescent lamp must also have a satisfactory lumen maintenance during life. The substitution of lighter iill gases such as neon for the usual argon lill vhas aggravated the problem of lumen maintenance insofar as sputtering occurs at a much faster rate resulting in a more pronounced blackening of the envelope. The use in the prior art highly-loaded lamps of a T12 (l1/2 inch diameter) bulb with a lighter gas ll has further aggravated the maintenance problem because of the more intense blackening of such relatively small bulbs. In the case of the grooved a larger T17 (2l/s inch diameter) envelope is employed. However, since the loading per unit area of envelope is not uniform because of the grooves, the same problem with regard to eliiciency and maintenance prevail.

It is accordingly the general object of this invention to provide a high output uorescent lamp that can be conveniently and inexpensively manufactured and which has a higher efficiency and better lumen maintenance than prior art lamps of the same loading.

The aforesaid object of the invention, and others which will become apparent as the description proceeds, are achieved by providing a fluorescent lamp having a fill gas and bulb size such that the operating temperature for optimum UV generation and eiciency is considerably below that heretofore considered standard for fluorescent lamps. Specifically, a neon or neon-rich fill gas and a high output lamp,v

v operated at an ambient of 40 3,252,028 Patented May 17, 1966 T17 envelope are combined according to a preferred embodiment of the invention to provide a high output lamp that not only has a higher etiiciency but better lumen maintenance during life than the prior art lamp designs. With this combination the operating temperature for optimum mercury vapor concentration is about 35 C. instead of the usual 40-45" 'C. Means for maintaining a region within the lamp that has the aforesaid lower operating temperature is also provided.

A better understanding of the invention will be obtained by referring to the accompanying drawing, wherein:

FIGURE 1 is an elevational view of a high output ll-uorescent lamp that incorporates this invention, a part of the envelope being omitted for convenience of illustration;

FIG. 2 is a graph illustrating the relationship between the voltage-versus-current characteristics and the bulb diameter for a plurality of liuorescentlamps of conventional loading and design; f

FIG. 3 is a graph illustrating the relationship between the fill gas composition and the relative efliciency and brightness of highly-loaded lamps embodying t-he invention;

FIG. 4 is a graph illustrating the relationship between the lamp wattage and the fill gas composition of the improved lamps; and

FIG. 5 is a graph illustrating the relationship 'between the end chamber temperature and the relative efficiency and brightness for a representative lamp incorporating the invention.

While this invention can be advantageously employed to improve the eiciency and maintenance of various kinds of discharge lamps, it is particularly adapted for use in conjunction with high output uorescent lamps and has accordingly been so illustrated and will be so described. Y

In FIG. 1 there is shown a highly-loaded fluorescent lamp 10 embodying this invention which lamp generally comprises a tubular light-transmitting envelope 12 that is provided at each end with the usual bases 13 and 14. Mount assemblies 15 and 16 are sealed to each end of the envelope and support oppositely disposed thermionic elect-rodes 17 and 18, such as tungsten coils that have been coated with a suitable electron-emissive material such as the well-known mixture of alkaline earth oxides, for example. with phosphor 22 that converts the UV radiation generated Within the discharge into visible light. The envelope also contains the usual charge of mercury 20 and an inert ionizable lill gas. The iill gas in accordance with this invention comprises neon, or a preselected mixture of neon and argon, at'a preselected pressure, as hereinafter disclosed.

Suitable cooling means for controlling the mercury vapor pressure is provided, such as heat-deflecting shields 23 and 24 that are held in transverse relation with respect to the lamp axis by the mount assemblies 15 and 16. The shields may be fabricated from a suitable metal such as nickel and are of such diameter that they provide a cooling chamber of predetermined length L at each end of the lamp 10.

As shown in FIG. 2, the character of a mercury-rare gas discharge is such that for a given lamp design and current density the lamp voltage inherently decreases as the diameter of the envelope increases. Thus, at a relative current density of about 50 a T8 lamp has a rating' of about volts (as shown by curve a rating of about 95 volts (curve 28), a rating of about 65 volts (curve 30). which these curves were obtained were Ventional loading and design filled with 26), a T12 lamp 2 mm. argon The inner surface of the envelope is coatedand a T17 lamp The lamps from lamps of con-- C. On the basis of such 3 data it was the general belief heretofore that T17 tubing was not desirable or practical for use in making highlyloaded lamps because of the low voltage gradient and low eliiciency which would normally result.

However, it has been discovered that envelopes having a diameter 'greater than 11/2 inches (T12) up to and including a T17 (2% inches) size cannot only be employed in high output fluorescent lamps but that a higher eicie'ncy and better lumen maintenance will be obtainedproviding the proper iil-l gas and pressure are used and a cooling chamber of suftieient size is employed. Specitically, it has been found that a T17 high output lamp of superior maintenancev and efliicency can be obtained in accordance with this invention by using afilling of 100% neon, or a mixture of neon and argon which contains at least 80% neon, and a ill pressure of between about 0.5 and 2.5 m'm. of mercury, and preferably from about 1.5 to 2.5 mm. For optimum eiiiciency and brightness the lill pressure should be `about 2 mm.

For some reasonV which is not perfectlyunderstood the laforesaid combination of a T17 envelope and `a specic ill gas composition and pressure lowers the temperature required for optimum mercury vapor pressure and UV generation by about 5-10 C. That is, instead of maximum eficiency occurring at an operating temperature of 40-45 C., 'as in the case with conventional and other types of high output lamps, the T17 lamp 10 of the present invention operates most eiiiciently when there is a region within the lamp that has a normal operating ternperature of from about 30 to 35 C.

According to the illustrated embodiment of the invention, the required lower operating temperature for optimum eiiiciency is obtained by correlating the diameter D of the envelope l2 and the length L of the cooling end chamber. In thespeciic example of a T17 bulb here illustrated where D is 2% inches, it has been found that a cooling chamber having a length L of 7 centimeters provided the required lower temperature and maintained the mercury vapor pressure at the correct value when the lamp was operated at its rated loading. The lamp in this case had an overall length of 4 feet and operated at 100 watts, that i's, at a loading of 25 watts per foot.

As is shown by the solid line curve 32 of FIG. 3, the brightness or light output of the aforesaid T17 lamp decreases as higher concentrations of argon are added to the neon lill gas. However, as indicated by the dottedline curve `34, the eliiciency vslowly increases as small amounts of argon are added, reaches a maximum at about 90% neon-10% argon, and then rapidly decreases with higher concentrations of argon. The lill gas, according- 1y, consists of 100% neon, or a mixture of neon and argon wherein neon constitutes at least 80%y of the mixture, .and preferably from about 85 to 95% thereof. The lamps on which these curves are based were 100 Watt T17 lamps 4 foot long having an end chamber temperature of approximately 35 C. and a lill pressure of 2 mm. Points A and B designate the eliiciency and brightness,

respectively, of a lamp of the same construction but with a conventional 100% argon fill at 3.4 mm. of pressure. From FIG. 3 it will be seen that at the. prescribed lower operating temperature, a highly-eflicient T17 lamp having a loading of 25 watts per foot can be provided which will operate at optimum efficiency when the till gas comprises a neon-argon mixture in the ratio of approximatel 9/ 1.

yAs shown in FIG. 4, the wattage of a T17 lamp of the same design as described above decreases rather rapidly from a maximum when 100% neon is used as the fill gas and then begins to level out when the iil-l gas comprises a 9/1 mixture of neon and argon. With respect to maximum wattage and brightness, optimum results will accordingly be realized with 100% neon, or a mixture of neon and argon wherein the argon is present in only very small amounts. In contrast, a T 17 of the same construction but having a conventional 100% argon lill at 3.4 mm.

4 pressure operates at a much lower wattage as indicated by point C in FIG. 4.

As will be noted from the dotted-line `curve 38 of FIG. 5, when a neon-10% argon gas mixture at 2 mm.' pressure is employed in the T17 highly-loaded lamp of this invention the eficiency is at a maximum when the end chamber temperature is about 34 or 35 C. Moreover, fas is evident from the solid line curve 40, the brightness or light output is at a maximum when the end chamber temperature is about 3l to 32 C. Thus, both the efficiency and brightness are at a maximum at approximately the same end chamber temperature and mercury vapor pressure. This is rather surprising since this is not the case in any other highly-loaded lamp, or any other type of iluorescent lamp for that matter.

As an example of the order of improvement attainable with highly-loaded lamps embodying this invention, the light output and eiciency as well as the power factor of a 4 foot T17 lamp having an 87% neon-13% argon till are compared below in Table I with .a prior art T12 80% neon-20% argon highly-loaded lamp after 100 hours burning. In each case, the lamps had end chambers at both ends 7 centimeters long and were provided with heat shields behind the electrodes.

TABLE I Lamp Construc- Cur- E., Power tion rent, Volts Watts Lumens LPW Factor amps T12, 80 Ne-20 AL l. 5 86 109 7, 200 6G 85 T17, S7 Nts-13 Atl. 5 75 102 7, 350 72 91 As will be seen from the foregoing table, the highlyloaded lamp having a neon-rich fill gas mixture, a T17 envelope and a lowerv optimum operating temperature in accordance with this invention not only produces more lumens and has a higher eliiciency than the prior art lamp but also has a higher power factor, which latter feature is desirable since the actual lamp wattage is accordingly proportionately higher. Zero hour readings of the test lamps has Shown that the drop in light output of the improved T17 lamps dropped about 2 to 3% after hours burning Whereas a 9% drop occurred in the case of the prior art T12 lamps. Thus, a T17 highlyloaded fluorescent lamp having the specific designA features hereinbcfore described is superior in both efficiency and lumen maintenance in comparison to conventional highly-loaded lamps having bulbs of smaller diameter.

It `will be obvious that the lower operating temperature required for optimum eiciency can be obtained by means other thancooling end chambers. VFor example, thermoelectric cooling can be employed in the manner disclosed in U.S. Patent No. 2,932,753.

It will be apparent from the foregoing that the object of the invention has been achieved insofar as a highlyloaded iluorescent lamp has been provided which not only exhibits an improved efticiency and lumen maintenance during life but which can be very easily and economically fabricated insofar as it does not require an envelope of complex configuration.

While a specilic lamp embodiment has been illustrated and described in detail by way of example, it will be appreciated that various modiiications therein and in the ll gas component can be made without departing from the spirit and scope of the invention.

We claim:

1. A low-pressure electric discharge lamp adapted to be operated at a predetermined wattage loading and comprising; a tubular radiation-transmitting envelope containing a pair of spaced electrodes, an ionizable medium within said envelope consisting of mercury and a till gas selected from the group consisting of neon and a mixture of neon and argon wherein neon constitutes at least about 80% of the mixture, the pressure of said iill gas being from about `0.5 to 2.5mm., and means associated with said lamp for providing a cool region therein, the diameter of said tubular envelope at said cool region being so correlated relative to the cooling ability of said cooling means that the latter provides a location Within said lamp that has a temperature of from about 30 to 35 C. when the lamp is operated at said loading.

2. An electric discharge lamp as set forth in claim 1 wherein said tll gas is 100% neon.

3. An electric discharge lamp as set forth in claim 1 wherein said ll gas consists of a mixture of neon and argon containing at least 80% neon.

4. A high-output low-pressure discharge lamp comprising; a tubular radiation-transmitting envelope having a diameter greater than 11/2 inches, a thermionic electrode at each end of said envelope, a charge of mercury and an ionizable till gas sealed within said envelope, said ll gas comprising essentially neon at from about 1.5 to 2.5 mm. pressure, and means integral with said lamp providing a cooling chamber therein, said lamp a predetermined wattage rating and the dimensions of said cooling chamber and the diameter of said envelope being so correlated that there is provided within said chamber a region having a temperature of from about 30 to 35 C. when the lamp is operated at its rated wattage.

5. A fluorescent lamp adapted to be operated at a loading in excess of 16 watts per foot and comprising; a tubular envelope having a diameter of approximately 2% inches, a phosphor coating on the inner surface of said envelope, a thermionic electrode at each end of said envelope, a contained charge of mercury and an inert ionizable fill gas, said fill gas comprising essentially neon at from about 1.5 to 2.5 mm. pressure, and a cooling chamber at one end of said envelope, the length of said cooling chamber being so correlated with respect to the envelope diameter that there is provided within said chamber a region that has a temperature of from about 30l to 35 C.

when the lamp is operated at said loading.

6. A fluorescent lamp as set forth in claim 5 wherein said fill gas consists of a mixture of approximately 90% neon and argon at about 2 mm. pressure.

7. A fluorescent lamp as set forth in claim 5 wherein said till gas consists of 100% neon at a pressure of about 2 mm. of mercury.

8. A low pressure electric discharge lamp adapted for operation at a predetermined wattage loading and comprising; a radiation-transmitting'envelope containing' a pair of spaced electrodes and a predetermined quantity of mercury, an ionizable lill ga-s including at least 80% neon sealed within said envelope, the pressure of said fill gas being from about 0.5 to 2.5 mm., and means adapted in conjunction with the cooling effect produced by the size of said envelope to provide a cool region within said envelope that has a temperature between about 30 and 35 C. when the lamp is operated at said wattage loading.

' virtue of its size to provide a 9. An electric discharge lamp as set forth in claim 1 wherein said cooling means comprises an energizable thermoelectric member.

10. A uorescent lamp adapted for operation at a loading greater than 16 watts per foot comprising; a tubular envelope having a diameter of approximately 2% inches, a thermionic electrode at each end of said envelope, a predetermined quantity of mercury and an ionizable ll gas comprising neon containing up to 20% argon sealed within said envelope, the pressure of said ll gas being from about 0.5 to 2.5 mm., and a yheat-deflecting shield located between one of said electrodes and the proximate end of said envelope at a point such that it denes, t0- gether with the encircling end portion of said envelope, a cooling chamber that is approximately 7 centimeters long and 2% inches in diameter and is adapted by virtue of its size to provide a region within said envelope that has a temperature of from about 30 to 35 C. when the lamp is operated at said loading.

11. A fluorescent lamp adapted for operation at a loading greater than 16 watts per foot comprising; a tubular envelope having a diameter of approximately 21/8 inches, a thermionic electrode supported by a mount assembly at each end of said envelope, a predetermined quantity of mercury and an ionizable ll gas comprising at least about neon at a pressure of about 2 millimeters sealed within said envelope, and a heat-detlecting shield on one of said mount assemblies and supported thereby in transverse relation with respect to the longitudinal axis of said envelope at a point located between the proximate electrode and the end wall of the envelope such that it defines, together with the encircling end portion of said envelope, a cooling chamber that is approximately 7 centimeters long and 21/8 inches in diameter and is adapted by region within said envelope that has a temperature of from about 30 to 35 C. when the lamp is operated at said loading.

References Cited by the Examiner UNITED STATES PATENTS 2,906,905 9/1959 wares 31a-.109 2,930,919 3/1960 Wainio 313-109 2,946,909 7/1960 Morehead 313-109 X 2,961,566 11/1960 Waymouth et al. 313-109 X FORElGN PATENTS 863,467 3/1961 Great Britain. 863,468 3/1961 Great Britain.

GEORGE N. WESTBY, Primary Examiner. RALPH G. NILSON, Examiner. C. R. CAMPBELL, Assistant Examiner. 

11. A FLUORESCENT LAMP ADAPTED FOR OPERATION AT A LOADING GREATER THAN 16 WATTS PER FOOT COMPRISING; A TUBULAR ENVELOPE HAVING A DIAMETER OF APPROXIMATELY 2 1/8 INCHES, A THERMIONIC ELECTRODE SUPPORTED BY A MOUNT ASSEMBLY AT EACH END OF SAID ENVELOPE, A PREDETERMINED QUANTITY OF MERCURY AND AN IONIZABLE FILL GAS COMPRISING AT LEAST ABOUT 90% NEON AT A PRESSURE OF ABOUT 2 MILLIMETERS SEALED WITHIN SAID ENVELOPE, AND A HEAT-DEFLECTING SHIELD ON ONE OF SAID MOUNT ASSEMBLIES AND SUPPORTED THEREBY IN TRANSVERSE RELATION WITH RESPECT TO THE LONGITUDINAL AXIS OF SAID ENVELOPE AT A POINT LOCATED BETWEEN THE PROXIAMTE ELECTRODE AND THE END WALL OF THE ENVELOPE SUCH THAT IT DEFINES, TOGETHER WITH THE ENCIRCLING END PORTION OF SAID ENVELOPE, A COOLING CHAMBER THAT IS APPROXIMATELY 7 CENTIMETERS LONG AND 2 1/8 INCHES IN DIAMETER AND IS ADAPTED BY VIRTUE OF ITS SIZE TO PROVIDE A REGION WITHIN SAID ENVELOPE THAT HAS A TEMPERATURE OF FROM ABOUT 30 TO 35*C. WHEN THE LAMP IS OPERATED AT SAID LOADING. 